conj (1)如果(in the likely event that;on condition that)(与现在时和完成时连用，表示很可能发生的情况)：I must leave～that's the case. 如果事情是那样，我必须离开。 I～(it is) true，this will cause us a lot of trouble. 如果这是真的，将会给我们带来很多麻烦。 I～it snows tomorrow，we will build a snowman. 如果明天下雪，我们就堆个雪人。 I～(it is) necessary，ring me at home. 如果有必要，给我家里打电话。He will come～(he is) asked. 如果要他来，他就会来的。You will forgive me～I have hurt you. 如果我伤了你的心，请你原谅。 I will return the book on Monday～I have finished it. 如果我看完了这本书，我星期一就还。

(2)假如(若)，要是(与过去的时态连用，表示虚拟的情况)(supposing)：I～I were you，I would go to night school. 我要是你，我就上夜校。 I～you had your choice，where would you go? 假如你可以选择，你要去哪儿? I～I knew her name，I would tell you. 我要是知道她的名字，我会告诉你的。You wouldn't be anywhere at all～it weren't for her. 要不是她，你将一事无成。I～you had asked me，I would have told you the whole story. 假如你问了我，我会把全部情况告诉你的。 I～it had not been for her，I might not have understood. 要不是她，我可能还不明白是怎么一回事。I～you were speaking for him，it would carry more weight. 假如你在代表他说话，你的话会更有分量。I should be more happy to go with you～I should not be in the way. 要是不妨碍你的话，我很高兴与你一起去。I～it hadn't been for the doctor's care，I should not be speaking to you now. 当时要不是医生的治疗，我现在就不能和你讲话了。

(3)请(与will 和would 连用，表示客气的请求)：I～you will come this way，the manager will see you now. 请您这边走，经理现在就见您。I～you will be kind enough to fill in this form，I'll have your baggage brought in. 请您填好这张表，我这就叫人把您的行李送来。Pass me that box，～you would. 请把那个盒子递给我。

(4)(每)当……时(when(ever))：I always feel ill～someone is smoking near me. 一有人在我附近抽烟我就感到不舒服。I～they are tired，they have a short rest. 他们累了就休息一会儿。I～he feels any doubt，he asks. 他一有疑惑就问。I～she wanted to shop，she would go to Paris. 每当要购物时她就去巴黎。

(5)是否(whether)(用于ask，know，find out，wonder等动词之后)：I asked her～she had any rooms to let. 我问她是否有房间要出租。I wonder～I can get some advice from you. 不知您是否能给我出点主意。She didn't say～she knew him or not. 她没有说是否认识他。

(6)即使，就算，虽然(even if;though)：I～I am wrong，you are wrong，too. 就算我错了，你也不对。You must come in，～only for a minute. 你一定要进来，即使一小会儿也好。It was a nice，～humid，day. 虽说湿气有点儿重，可还是个好天。His style，～mannered，is pleasant to read. 他文章的风格虽然有点矫揉造作，读起来还是令人愉快的。The profits，～a little lower than last year's，are still extremely healthy. 利润虽然比去年低一点，但仍然是丰厚的。

(7)这不是，可是(用于否定句，表示惊奇、沮丧等)：Well，～it isn't our old friend Smith! 哎，这不是我们的老朋友史密斯嘛! I～he isn't the laziest guy on earth! 他可是世界上最懒的家伙了! I～this isn't the best idea I've heard so far! 这可是到目前为止我听到的最好的主意!

if and when 如果(用来表示将来并不肯定的可能性)：I～and when we ever meet again，I hope he remembers to apologize to me. 如果我们还能重逢，我希望他别忘了向我道歉。

if anything 甚至可以说，更可能的是，相反：She was，～anything，more absent-minded than usual. 可以说，她比以往更心不在焉。Joe isn't a bad boy. I～anything，he's a pretty good one. 乔不是个坏孩子。相反，他是个很好的孩子。

if need be 如果有必要的话：He would work all day，and all night，too，～need be. 如果有必要的话，他愿意整天，甚至整夜地工作。

if only 但愿，要是……就好了(通常用过去的时态，表示愿望)：I～only they had enough sense of humour! 但愿他们有足够的幽默感! I～only it would stop raining! 但愿雨能停下来! I～we had only known. 要是当时我们知道就好了。

only if 只有(在句首时后面主句要倒装)：Only ～you study hard can you pass the exams. 你只有努力学习才能通过考试。These snakes only attack～they feel threatened.只有感觉受到威胁这些蛇才会攻击。

n 条件，不确定因素(condition;uncertainty)[C](infml)：The future is full of ～s. 未来充满着不确定因素。There are too many～s in his offer. 在他的提议中假设条件太多了。

ifs and buts 这样那样的借口：I don't want any～s and buts，swallow your medicine immediately. 我不要这样那样的借口，赶快把药喝下去。

【辨异】if 和whether均可用来引导一个间接一般疑问句，如：I'm not sure if/whether I'll have time.(我不能肯定我有时间。)当提出两种选择时，whether 更常用，特别是在正式文体中，如：Let me know whether you can come or not.(请告知我你是否能来。)whether后面可直接跟or not，if 则不可。某些动词(如discuss)后面只能跟whether，不能跟if，如：We discussed whether we should close the shop.(我们讨论了是否关闭这个店的问题。)whether可用在介词后，或带to的不定式前，if 则不能这样用，如：I haven't settled the question of whether I'll go back home.(我还没有决定是否要回家。)She doesn't know whether to get married now or wait.(她不知道是现在结婚还是再等一等。)引导主语从句时只能用whether，如：Whether they win or lose is all the same to me.(他们是赢还是输对我都一样。)